Machine Gun Kelly's finally living up to his moniker ... walking into the studio with a rifle slung over his shoulder.

The rapper was spotted walking into a building in Hollywood on Monday ... wearing a black leather jacket with matching shorts, combat boots, and a cap.

Also matching the look ... a similarly colored rifle at his side -- the kind of thing Tony Montana might use to make his last stand.

Unclear if this is real or just a prop ... we've reached out to MGK's side for more information.

BTW ... this is the very same recording studio outside of which Kelly met up with his rumored GF, Dakota Johnson, earlier this month.

Remember ... we spotted the two hopping into his car -- with MGK opening the door for the lovely lady. It seems like they're dating ... though they have not officially confirmed their new romance.

MGK and Dakota probably don't want to jump the gun on announcing their new relationship ... though we know they're both straight shooters -- so don't be surprised if they clue the public in officially soon!